Wired radio on power lines



Oct. 23, 1928.

R. D. DUNCAN, JR

WIRED RADIO ON POWER LINES Filed March l2, 1925 INVENTOR effa/fa v BY M?0%M1WM ATTORNEY` Patented Oct. 23, 1928.

UNITEDI STATES PATENT OFFICE.

ROBERT D. DUNCAN, J' R., 0E EAST ORANGE, NEW' JERSEY, ASSIGNOR TO WIREDRADIO, -IN C., OF NEW YORK, N. Y., A CORPORATION 0F DELAWARE.

winni) RADIO oN POWER LINES.

Application led March 12 1925. Serial No. 14,856.4

This invention has to do with the transmission of high frequency carriercurrent for signaling purposes over electric power lines-especiallywired radio broadcastingand has vfor its object to modify the'charac--teristics, at high frequencies, of powervdistribution transformerswhich, under some conditions, are found to be deficient in their abilityto transmit high frequency currents, and thereby to obtain more uniformdistribution of the carrier current through the entire power network.

Experience has shown that in electric power distribution systems, onwhich high frequency currents are superimposed as in wired radiobroadcasting, that as between subscribers about equally distant from thecentral power station, at which the carrier current is impressed on thesystem, there is often a large difference in the strength of signalsreceived. This condition has been found to be due, in some cases, to thefact that certain power transformers,4 not being designed with a view topassing high frequencies, have such high input impedance as viewed fromthe transmission line that very little high frequency current can pass.

through the turns of the primary windings, and, accordingly, there isfailure to induce suflicient high frequency current into the secondarywindings. This condition may be cured by the use of high frequencyby-passp ing circuits between the primary and secondary windings but todo so it is necessary to employ expensive condensers,` properly housedand mounted. Ordinarily two condensers would be required for each trans-.inductance is preferably so chosen that the effective overall reactanceat the high frequency dealt with, of the combined primary radiooperation over electric lighting svsi tems, with the consequent absenceof coinpreliensive experimental data, the high frequency deficiencies ofvarious types and sizes of distribution transformers have had to bedetermined by actual receiving tests at subscribers stations served bydifferent transformers. Having thus determined the existence ofdeficiencies, a subsequent study and determination of the high frequencycharacteristics of the transformers-has disclosed the reasons for suchdeficiencies .and yielded the present method of correcting therefor.

For a more comprehensive explanation and description of the inventionreference will now be had to the accompanying drawing, in which, f

Fig. l is a diagrammatic illustration of a power transmission linehaving a traiisformer bridged across it and on which itis assumed thereis superimposed a high frequency carrier current. Figs. 2, 3 4and 4; arereactance and resistance curves,

Fig. 5 is an equivalent circuit diagram illustrating the character ofthe high frequency impedance of the primary winding of `the transformerof Fig. 1,

In Fig. 1, 1 and 2 are the two conductors,

respectively, of a power transmission line carrying low frequency powercurrent generally at a relatively high voltage and high frequencysignaling current or carrier current. 3 is the primary winding of apower transformer 4, and is bridged across the line. The secondarwinding 5 is connected to local distribution ines 6, 7 and 8 and at itsmid point is usually grounded as indicatedat 9. Across the lines 6, 7and 8 are shown lamps or other power consuming devices 10 and wiredradio receiving apparatus 11.

The high frequency ,current` I, which'will flow through the primarywinding 3, due to the high frequency voltage E. across the terminals ofthe primary winding is given by (1) :i--ZE where Z is the inputimpedance of the transformer eective at the particular high frequencydealt with. The lower the value of Z,

the greater that of I, for a constant E.

. relatively low positive value, that is, inductive reactancepredominates-increasing with increase of frequency to a positive maximumvalue, and drops through zero to a maximum negative value, that is,capacitive reactance which with further increase of frequency approacheszero asymptotically.

The frequency fo may be of the order of several thousand cycles, itsexact value depending upon the size of the transformer, its loadingandother factors.

The resistance component of the impedance as a function ofthe frequencyis given by curve 13 of Fig. 3 and, as indicated, is alwayspositiveattaining a high value at fo.

At any frequency f, the impedance, is expressed by and since a2 isnegative, that'is, capacitive,

ordinarily dealt with: in a ,certain one, kilowatt transformer, at afrequency of 40,000 cycles r=7,000 ohms; m=53,000 ohms; Z= 53,500 ohms.

From Equation 1) it is obvious that an increase in current I may be hadby a decrease in the value of Z. With a constant frequency, theimpedance Z may be decreased by decreasing either of its components r orThe latter offers vthe most immediate possibilities. By adding, inseries with the primary winding 3, an auxiliary inductance as shown inFig. 6, the capacitive reactance is either wholly or in part offset. InFig. 6 the auxiliary inductance is represented by reference numeral 16.

The manner of obtaining a reduction in the capacitive reactance isillustrated by curves 12 and 14 of Fig. 2 and curve 15 of Fig. 4.

The reactance of a pure inductance is given by (4) 1E" 2 'lrfL where fis the frequency and L the inductance. This is the equation, of astraight line with w and f as the variables and is represented by curve14 of Fig. 2. The effective reactance of the circuit 16, 3 of Fig. 6 isthe algebraic sum of the ordinates of curves 12 and 14 which yield curve15 of Fig. 4. Then in Fig. 4,

Again, viewed from the standpoint of lines 1, 2, the circuit of Fig. 6has the characteristics o f and may be represented by the equivalentcircuit of Fig. 7 The impedance of this circuit is Utilizing the valuespreviously given wherein m=53,000 ohms and lassuming that it is desiredto reduce this by it is obvious that m =26,500 ohms.

From (4) L= 0.1055 henrys Translated into simple language, this means,in the case of the transformer previously referred to, that thereactance of the primary winding at 40,000 cycles may be reducedone-half by the addition, in Vseries therewith, of an auxiliaryinductance of .1055 henrys. Assuming that the high frequency voltagedrop across the primary winding and auxiliary inductance in seriesremains unchanged, the high frequency currents passing through thetransformer would be approximately doubled.

In most alternating current power distribution networks, the linesextending from the power stations or sub-stations each have aconsiderable number of transformers, some of which are much moreefficient than others in passing high frequency carrier currents. It hasbeen found, however, that the majority of such transformers are capableof passing sufficient high frequency current to require no modification,that is'- to say, they do not require the addition of auxiliaryinductance. In fact, it is found that usually only a small percentage ofthe transformers require such treatment. As a rule it is not desirableto modify the high frequencyimpedance of a deficient transformer' sothat it will pass the maximum possible amount of highI frequencycurrent, but only to such an extent that it will pass whatever isregarded as the necessary amount to effect satisfactory reception atevery subscribers station served.

The auxiliary inductance should, of course, be constructed to carrysafely the power current which may be in the neighborhood of amperes.

It is believed that in no case would the reactance of an auxiliaryinductance, such as would be required, be suiiiciently high tomaterially affect the power current.

I claim:

1. A wired radio system comprising a power transmission line for thedelivery of power and high frequency signaling energy to a plurality ofsubscriber stations, apower transformer lat each of said subscriberstations, said power transformer having primary and secondary windingsfor delivery of said power to a load circuit, an auxiliary inductancedevice interposed in series with said primary winding and connectedacross said transmission line, the combined impedance of said primarywinding and said auxiliary inductance at the frequency of said signalingenergy being less than the impedance of said primary winding alone atthe same frequency said auxiliary inductance being designed to pass saidpower to said'load simultaneously with the delivery of said highfrequency signaling energy to said load through said transformer.

2. A wired radio system comprising a transmission line forsimultaneously conveying power and high frequency signaling energy to aplurality of subscriber stations, aV plurality of power transformerseach having primary and secondary windings, with the primary windingsthereof connected in bridge to said line, a load circuit connected tosaid secondary windings including power .consuming devices and wiredradio receivmg apparatus, and means for substantially equalizing thesupply of high frequency signaling energy to said wired radio receivingapparatus at each of said subscriber stations said means comprising aninductance device connected in series with the primary winding ofselected power transformers and in bridge to said line, said inductancedevices each having such value that the combined impedance ofassociatedprimary windings and said inductance devices at the frequencyof said signaling energy is substantially less than the impedance ofsaid primary winding alone at sa1d frequency whereby said signalingenergy may be transferred to said wired radio receiving apparatusatlrelatively large amplitude simultaneously with the supply of power tosaid load circuit through said transformers.

In testimony whereof I aiix my signature.

r ROBERT D. DNCAN, JR.

